1. Field of the Invention
The present invention relates to biodegradable fibers. More precisely, the invention relates to biodegradable fibers of polylactic acid that provide biodegradable plastics which are more environmentally acceptable than conventional plastics, and which have a controllable rate of biodegradation which varies depending upon how the fibers are used.
2. Discussion of the Background
Polylactic acid is one of the recyclable plastics presently recognized as having good environmental qualities. Polylactic acid fibers made from polylactic acid are expected to be commercialized as biodegradable fibers that are environmentally xe2x80x9cfriendlyxe2x80x9d and ecologically favorable.
In order to become commercially acceptable, biodegradable fibers must a level of mechanical strength which is at least that of conventional fibers such as polyester fibers. Specifically, biodegradable fibers are not commercially acceptable if they biodegrade during ordinary use. Accordingly, various proposals have been made relating to the production and the physical properties of biodegradable fibers which have commercially acceptable properties.
For example, one method proposed for efficiently producing polylactic acid fibers having stable physical properties comprises melt-spinning polylactic acid fibers, cooling and solidifying the fibers, then re-heating and exposing the heated fibers to a stream of air. It is claimed that this method promotes the orientation and the crystallization of the fiber-forming polymer, and provides fibers having high levels of mechanical strength and elasticity, which are not provided by any ordinary high-speed fiber-spinning and drawing method (for example, see JP-A-11-131323, paragraph 0016 and FIG. 1).
Another method proposed for producing polylactic acid fibers comprises using a hydrolysis-resistant polymer having a reduced amount of low-molecular weight compounds, in order to stabilize the fibers in the natural environment, especially in water or under humid conditions (for example, see JP-A-7-316272, paragraph 0002, lines 1 to 5, and paragraph 0005; and JP-A-9-21018 paragraphs 0006 and 0007).
These methods are intended to retard the biodegradation of the fibers while in use, but they do not promote the biodegradation of the fibers after use, and do not control or retard the rate of biodegradation of the fibers while in use. In addition, the biodegradability of the fibers and nonwoven fabrics made from these fibers is such that they may lose their strength in a period of from a half year to one year after they are buried in the ground. This means that fibrous wastes of this type require a long period of time (i.e., on the order of a year) before such buried wastes are biodegraded. Accordingly, such fibers and fabrics are unsuitable for land reclamation. On the other hand, incinerating such fibers and fabrics is undesirable because it may promote global warming.
The comparative examples of the biodegradation-retarding methods shown in various patent publications provide some examples of biodegradation-promoting methods, but do not describe methods of controlling biodegradationxe2x80x94that is, ensure the stability of biodegradable fibers while in use and rapidly biodegrade the used fibers after disposal.
Various methods of promoting and controlling the biodegradation of articles have been investigated (for example, see JP-A-9-263700, paragraph number 0011). This method comprises adding from 10 to 40% by weight of dry coconut powder to a polymer followed by shaping the mixture into articles. When articles produced by this method are buried in the ground after use, the dry coconut powder absorbs water in the ground, and the buried articles are thereby swollen and biodegraded. This method is unique in that it provides biodegradation control of buried articles. However, the size of the particles of coconut powder mixed with the polymer is large, from 20 to 80 xcexcm. Thus, this method is not suitable for fibers having a diameter of only from 14 to 30 xcexcm.
Fibers having a core-sheath structure, composed of polymers having different biodegradability, or fibers having a notched surface structure composed of different polymers have been proposed (for example, see JP-A-9-78427, paragraph number 0014 and Japanese Patent No. 3,304,237, paragraph number 0006). These fibers have a polymer of lower biodegradability supporting another polymer of higher biodegradability, thereby preventing the fibers from deteriorating while in use. However, the biodegradation of the polymer of higher biodegradability varies depending on the surrounding conditions, and therefore the biodegradation of the fibers themselves varies depending on the surrounding conditions. Thus, the biodegradation of the fibers cannot be controlled independently of the conditions surrounding the fibers. Accordingly, the life of products made from such fibers varies depending on the environment in which they are used, and the biodegradation of such fibrous products is not promoted at all after disposal. In other words, this method does not provide control of the biodegradation of the fibers.
Similarly to the present invention, a method of notching the surfaces of fibers has been proposed. This method comprises thermally stretching fibers to a draw ratio not lower than the maximum draw ratio, to thereby form uniform voids inside the fibers. In addition, the surfaces of the stretched fibers have streaky notches (see JP-A-11-293519, paragraph 0013, and photographs). However, these fibers take a long time, approximately 18 months, before they are actually biodegraded, as shown by the examples provided in this patent publication, and this method of biodegradability control does not conform to conventional practices of processing and biodegrading used fibers.
The biodegradable fibers that are most preferred for commercial use are those that maintain their strength while in actual use, but can rapidly degrade after use and disposal, or those having controllable biodegradability. Up to the present, however, no one has proposed controlling biodegradation, or provided biodegradable fibers based on controlling biodegradation.
The present invention solves the problems noted above and provides biodegradable fibers having physical properties suitable for practical use, and permits the time of biodegradation to be controlled in any desired manner.
The present inventors have found that, when fibers are designed to have a specific structure or when a fiber-processing agent is applied to the fibers, then the biodegradation of the fibers is retarded or promoted. Specifically, cracks are formed in the surface of the fibers so that the strength of the fibers is sufficient for ordinary practical use. On the other hand, when used articles comprising the fibers are disposed of, or formed into compost, the fibers are actively biodegraded after a specific processing agent is applied thereto. Accordingly it is possible to easily control the biodegradation of the fibers in any desired manner. The present invention provides biodegradable fibers of polylactic acid having a number-average molecular weight of from 50,000 to 150,000, wherein the alkali solubility of the inside part of each fiber is greater than that of the outer, peripheral portion of the fiber, and the surface of each fiber has from 5 to 50 cracks/10 cm.
Preferably, a fiber-processing agent having a pH of less than 7.8 is applied to the biodegradable fibers.
Also preferably, a fiber-processing agent (a), having a strength reduction-promoting constant (KR value) represented by the following formula (1) of less than 1.2, is applied to the biodegradable fibers.
Strength Reduction-promoting Constant(KR value)=TA/TBxe2x80x83xe2x80x83(1)
TA is the strength of fibers of polylactic acid having a number-average molecular weight of from 50,000 to 150,000, the fibers being prepared so that the alkali solubility of the inside portion (i.e., the core) of each fiber is greater than that of the outer, peripheral portion (i.e., the skin) of the fiber, and the surface of each fiber has from 5 to 50 cracks/10 cm. The strength of such fibers is measured after the fibers are degreased and then left at a temperature of 50xc2x0 C. and a humidity of 65% for 7 days.
TB is the strength of fibers of polylactic acid having a number-average molecular weight of from 50,000 to 150,000, the fibers being prepared so that the alkali solubility of the inside portion of each fiber is larger than that of the outer peripheral portion of the fibers, and the surface of each fiber has from 5 to 50 cracks/10 cm. The strength is measured after the fibers are degreased, from 1 to 5% by weight of the fiber-processing agent (a) is applied to the fibers held under a tension of from 0.05 to 0.20 g/dtex, and then the thus-processed fibers are left at a temperature of 50xc2x0 C. and a humidity of 65% for 7 days.
The invention also provides a method of controlling the biodegradation of the fibers and promoting the biodegradation of the biodegradable fibers, comprising treating the surfaces of the fibers with an alkaline fiber-processing agent having a pH of not lower than 7.8.
Preferably the fiber-processing agent (b) has a strength reduction-promoting constant (KR value), represented by formula (1) above, of at least 1.2, and is applied to the biodegradable fibers to promote the biodegradation of the fibers.
Also preferably, a processing agent that contains at least 1% by weight of at least one component selected from a group consisting of polyoxyethylene phosphates, phosphate salts, phosphate amines and oleic acids may be applied to the biodegradable fibers in order to promote the biodegradation of the fibers.
The present invention also provides a fabric formed from the biodegradable fibers.